This invention relates to method and apparatus of stabilizing uniformity of a static magnetic field in a magnetic resonance imaging apparatus which makes use of a nuclear magnetic resonance (hereinafter simply referred to as "NMR") phenomenon to obtain a tomographic image of a desired part of an object under examination (human body).
The magnetic resonance imaging apparatus utilizes the NMR phenomenon by applying a static magnetic field and gradient magnetic fields to an object to be examined to measure a density distribution, a relaxation time distribution and the like of a nuclear spin at a desired examination part of the examined object and display an image of a desired slice of the examined object on the basis of measured data.
A static magnetic field generating magnetic circuit serving as means for generating the aforementioned static magnetic field has a specific construction as shown in FIGS. 6 and 7, having a pair of permanent magnets 21a and 21b opposing to each other to form a gap A into which an object 1 under examination is insertible, yokes 22a and 22b for supporting and magnetically coupling the permanent magnets 21a and 21b, columns 23 for coupling the yokes 22a and 22b, and magnetic pole pieces 24a and 24b respectively secured to the opposing surfaces, confronting the gap A, of the paired permanent magnets 21a and 21b and taking the shape of a disk-like magnetic member having an annular projection formed at its periphery, whereby a magnetic field is generated in the gap A. The magnetic pole pieces 24a and 24b are adapted to improve uniformity of the static magnetic field and made of a ferromagnetic material. FIG. 7 is a sectional view taken on the line I--I of FIG. 6, depicting gradient magnetic field coils designated at reference numeral 9.
With the magnetic resonance imaging apparatus constructed as above, when the static magnetic field in the apparatus is non-uniform, a tomographic image of the examined object 1 will be distorted. In other words, the better the uniformity of the static magnetic field, the more the distortion of a tomographic image obtained is lessened. Accordingly, in the past, during installation, maintenance or inspection of the magnetic resonance imaging apparatus, mechanical configuration and positional relation in the construction shown in FIGS. 6 and 7 are adjusted to achieve adjustment of uniformity of the static magnetic field.
Actually, however, when a phantom or an examination object used for adjustment and evaluation of the apparatus is imaged after completion of the adjustment of uniformity of the static magnetic field in the prior art magnetic resonance imaging apparatus, the resulting image is sometimes distorted. For example, when a phantom 25 of a lattice pattern as shown in FIG. 8 is set in the gap A shown in FIG. 7 and imaged, a tomographic image I as shown in FIG. 9A which the phantom 25 imaged without distortion in shape should take is sometimes distorted in effect, resulting in a tomographic image I' as shown in FIG. 9B. Conceivably, this is due to the fact that the gradient magnetic fields applied when imaging the phantom change the magnetization intensity of the magnetic pole pieces 24a and 24b forming part of the static magnetic field generating magnetic circuit, so that the uniformity of the static magnetic field is lost and the non-uniformity results.
The magnetic pole pieces 24a and 24b are made of the ferromagnetic material which has in general a magnetic hysteresis. An example of its hysteresis curve is illustrated in FIG. 10. In the figure, abscissa represents magnetic field intensity H externally applied to the ferromagnetic material and ordinate represents magnetization intensity M of the ferromagnetic material applied with the magnetic field intensity H. The magnetization intensity M of the ferromagnetic material changes with the magnetic field intensity H applied thereto to take values on the closed magnetic hysteresis curve shown in FIG. 10.
With the prior art magnetic resonance imaging apparatus, once the adjustment of the static magnetic field uniformity has been finished by adjusting the mechanical configuration and positional relation in the construction of the static magnetic field generating magnetic circuit upon installation, maintenance and inspection of the apparatus, ordinarily imaging is conducted. During each imaging operation, gradient magnetic fields of variously different intensity values are applied for variously different intervals of time. As a result, the magnetization intensity M of the magnetic pole pieces 24a and 24b taking, for example, a point A on the curve of FIG. 10 immediately after completion of the adjustment of the static magnetic field uniformity changes from the point A to, for example, a point B through a subsequent imaging operation and then changes from the point B to, for example, a point C through a further subsequent imaging operation. Thus, with the prior art magnetic resonance imaging apparatus, the magnetization intensity M of the magnetic pole pieces adapted to make uniform the static magnetic field changes along a loop L1 which is subject to a relatively large change, thereby preventing the static magnetic field uniformity from being stabilized. In consequence, the resulting tomographic image of the examined object is sometimes distorted and excellent diagnostic images cannot be obtained, thus degrading efficiency of diagnosis.